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CHAPTER 24:
Capacitance, Dielectrics, Electric Energy Storage
Responses to Questions
1.
Yes. If the conductors have different shapes, then even if they have the same charge, they
will have different charge densities and therefore different electric fields near the surface.
There can be a potential difference between them. The definition of capacitance
C = Q/V
cannot be used here because it is defined for the case where the charges on the two
conductors of the capacitor are equal and opposite.
2.
Underestimate. If the separation between the plates is not very small compared to the plate
size, then fringing cannot be ignored and the electric field (for a given charge) will actually
be smaller. The capacitance is inversely proportional to potential and, for parallel plates, also
inversely proportional to the field, so the capacitance will actually be larger than that given
by the formula.
3.
Ignoring fringing field effects, the capacitance would decrease by a factor of 2, since the
area of overlap decreases by a factor of 2. (Fringing effects might actually be noticeable in
this configuration.)
4.
When a capacitor is first connected to a battery, charge flows to one plate. Because the
plates are separated by an insulating material, charge cannot cross the gap. An equal amount
of charge is therefore repelled from the opposite plate, leaving it with a charge that is equal
and opposite to the charge on the first plate. The two conductors of a capacitor will have
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 Spring '09
 Thomas
 Capacitance, Charge, Energy

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